Switching amplifiers, class-D amplifiers, PWM amplifiers, etc., are amplifiers that work by in principle multiplying the input utility signal, e.g. an audio signal, with a power signal, e.g. a DC voltage. They have many advantages and are becoming more and more popular in both consumer and professional markets as their quality increase.
Some of the disadvantages, however, that still exist with the known switching amplifiers are related to the way power is supplied to the amplifier. In order to be able to produce a full scale amplified audio signal, the switching power stage in previous switching amplifiers is continuously supplied with a maximum DC voltage, also when the utility signal is small or silent, or when the volume is decreased. The continuous high DC voltage even at idle times causes unnecessary much noise and EMI problems as the noise is proportional with the supply voltage, and the power consumption becomes unnecessarily high as most of the loss is proportional to the square of the supply voltage.
Some previous amplifiers solve some of the problems by letting the volume control, which normally scales the utility signal, act on the power supply voltage instead.
Another approach is to use a so-called tracking power supply, which varies the supply voltage in accordance with the utility signal by some predefined rule. The concept theoretically works because the product of a certain utility signal value and the DC supply voltage can also be achieved by multiplying a higher utility signal value with an accordingly lower supply voltage. Thus, by always decreasing the DC supply voltage to a value where a corresponding increase in utility signal value approaches the clip value, the highest possible utility signal value is always multiplied with the lowest possible supply voltage, and noise and power dissipation are reduced. Whereas it is relatively simple to establish a DC supply voltage that “tracks” the input utility signal, a sufficiently accurate corresponding correction of the utility input signal represents a problem.
For a certain kind of switching amplifiers, namely amplifiers with an error correcting feedback loop, a solution for implementing a tracking power supply is described in PCT Patent Application No. WO 2005/036731 A2. In this patent application is disclosed a power supply that delivers a DC supply voltage in some predefined correspondence with the input signal. Obviously, the varying supply voltage causes distortion of the amplifier output signal, in the same way as other power supply errors such as ripple, etc., but because of the error suppression enabled by the feedback loop, the input signal is automatically adjusted inversely.
However, none of the embodiments disclosed in the above-mentioned patent application works with amplifiers that do not comprise an error suppression feedback loop as described. For such amplifiers, the so-called open-loop switching amplifiers, there has so far not been any way to benefit from the advantages of utilizing tracking power supplies.
It is therefore an object of the present invention to enable the use of a tracking power supplies with open-loop switching amplifiers.